The association of aging with increased incidence of breast cancer is well known, but little understood. Emphasis has been placed on differences among tumors as a function of age, while considerably less attention is given to changes that occur normally in mammary epithelium during the process of aging, which may facilitate tumorigenesis. A commonly invoked mechanism to help explain age-related cancer is the gradual accumulation of mutations and epigenetic changes. However, it is known that cells harboring even the strongest oncogenes can appear phenotypically normal when held in check by normal tissue architecture. Most tissues exhibit functional and regenerative decline with advancing age. Our over-arching hypothesis is that age-associated breast cancer may partly result from loss-of-function alterations, e.g. changes to structural and architectural gatekeepers that maintain normal tissue organization and polarity, which leads to deleterious imbalances of or changes in the activity of progenitors and more differentiated epithelial lineages. That the majority of women live healthy cancer-free lives suggests age-related changes to the breast tissue are usually benign, but in cases when deleterious genomic changes also are present the combination with deteriorating microenvironments may be catastrophic. Some age-associated changes in breast includes increased fat and estrogen receptor expression; decreased connective tissue, numbers of alveoli and overall breast density; changes in collagen-type expression, and discontinuities in the basement membrane of the mammary gland. We simply do not know what impact these changes have on epithelial cell biology or on the architecture and organization with the gland. This proposal will use the Human Mammary Epithelial Cell (HMEC) Aging Resource, which is a large collection of normal finite-life span HMEC strains that were established from patients ranging in age from 16-91, to perform heretofore impossible quantitative and functional analysis of normal HMEC as a function of age. Changes in populations and functional properties of stem, progenitor, and more differentiated lineages will be measured, as will the ability of HMEC to form and maintain a normal organized bilayered architecture, an ability that is lost early in tumorigenesis, is affected by the aging process. In parallel with our functional analysis, we will perform an automated quantitative analysis of over 500 histological sections of normal breast to generate an atlas of aging-associated changes in the breast tissue, and to identify changes in organization in vivo among the different HMEC lineages. Our goal is to identify potentially deleterious changes that occur in most women during the aging process that could be targets of future prophylactic and preventative strategies.